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Sedimentology and biological assemblages

Sequences recovered at the Tahiti sites comprise carbonate and volcaniclastic sediments and rocks that are described below.

Modern reef sediments

Modern reef sediments consist of several decimeters of skeletal sands and gravels composed of rhodoliths, bioclasts rich in Halimeda segments, mollusk fragments, benthic foraminiferal tests, and nongeniculate coralline algal thalli. They include clasts of Halimeda packstone and coral clasts (especially branching and encrusting colonies) that are commonly encrusted with nongeniculate coralline algae and worms. The surfaces of some clasts are extensively bored and display brown staining. Cores of the rhodoliths generally correspond to coral fragments that in many instances exhibit traces of bioerosion.

Carbonate sequences

Two major carbonate sequences were identified based on their lithological features. Although dating results were not available before the writing of this chapter, the upper carbonate sequence is suspected to correspond to the last deglaciation (referred to as the last deglacial sequence [lithologic Unit I]), whereas the lower sequence is likely to be of older Pleistocene age (referred to as the older Pleistocene sequence [lithologic Unit II]).

The top of the last deglacial carbonate sequence is characterized by an abundance of thin crusts of nongeniculate coralline algae. Extensive bioerosion, black to brown staining, and hardgrounds are common within the top 2–3 m of the sequence.

The last deglacial carbonate sequence is mostly composed of coralgal frameworks heavily encrusted with microbialites locally interlayered with skeletal sand and rubble rich in coral and algal fragments and volcanic grains. In a few cases, coral colonies and algal-coated coral clasts are embedded in a skeletal grainstone/​packstone rich in Halimeda segments associated with benthic and encrusting foraminifers, bryozoans, echinoids, and mollusks.

Corals are well preserved and form distinctive associations characterized by various morphologies determining distinctive framework internal structure: branching, robust branching, massive, tabular, foliaceous, and encrusting (see descriptions in the “Sedimentology and biological assemblages” sections in the individual site chapters). Robust branching (e.g., Pocillopora and Acropora) and, to a lesser extent, tabular (e.g., Acropora) corals are usually thickly encrusted with nongeniculate coralline algae and microbialites to form dense and compact frameworks; vermetid gastropods and serpulids are locally associated with coralline algae. Foliaceous and encrusting coral colonies (e.g., Montipora, Porites, Pavona, Leptastrea, Psammocora, Astreopora, and agariciids and faviids) are thinly coated with nongeniculate coralline algae and microbialites to form loose frameworks. Large primary cavities in coralgal frameworks are partially filled with skeletal sands and gravels, including coral and coralline algal fragments, and with Halimeda segments; volcanic elements are generally subordinate. Cavity walls are commonly veneered with microbial dendritic fabrics.

Microbialites are abundant and represent the major structural and volumetric component of the reef rock. They develop within the primary cavities of the reef framework, where they generally overlie coralline algal crusts to form dark gray massive crusts as thick as 20 cm; they also develop in bioerosion cavities. Microbialites generally comprise a suite of fabrics, including two end-members represented by laminated fabrics and thrombolitic to dendritic accretions; thrombolites usually represent the last stage of encrustation.

Coralgal frameworks are commonly associated or interlayered with skeletal limestone and/or loose skeletal sediments, including coral and algal rubble and skeletal silt to sand.

Skeletal limestone is composed of skeletal packstone to floatstone rich in Halimeda segments and fragments of corals, coralline algae, and mollusks.

Skeletal sand is composed of fragments of corals, coralline and green (especially Halimeda) algae, and, to a lesser extent, bryozoans, echinoids, mollusks, and foraminifers (Amphistegina and Heterostegina).

Rubble consists mostly of gravels, pebbles, and cobbles of coral fragments (mostly of branching colonies and, to a lesser extent, tabular colonies) usually displaying encrustations with coralline algae and microbialites, crusts of nongeniculate coralline, chunks of microbialite crusts, and clasts of coralgal-microbialite frameworks. Halimeda segments, fragments of bivalves and gastropods, and rhodoliths also occur. Fragments are generally well rounded to subangular and display abundant traces of bioerosion. Some intraclasts display bioeroded surfaces and encrusting worm tubes. Volcanic pebbles are locally mixed with carbonate elements.

The contact between lithologic Units I and II is characterized by an irregular unconformity typified by abundant solution cavities that are partly filled with unconsolidated skeletal and volcanic sand, including coralline algal branches, coral gravels, and Halimeda segments; some cavities are partly filled with stalagmite crusts. The top of the older Pleistocene sequence (Unit II) is locally characterized by multiple bored and encrusted surfaces (hardgrounds). Several successive unconformities occur in the upper part of the older Pleistocene sequence.

The older Pleistocene carbonate sequence mostly comprises three major distinctive lithological subunits that are usually closely associated:

  • Subunit IIA: Well-lithified skeletal packstone/​grainstone to floatstone/​rudstone rich in nongeniculate coralline algal crusts, coral fragments (e.g., branching colonies of Pocillopora and Porites, encrusting colonies of Montipora and Porites, and massive colonies of Porites), and Halimeda segments; the abundance of volcanic elements and limestone lithoclasts is variable. Abundant centimeter-sized cavities, as well as the dissolution of skeletal grains (Halimeda segments, fragments of mollusks, and corals that form the core of rhodoliths) and the recrystallization of coral skeletons, indicate that these limestones were subject to subaerial diagenetic processes.
  • Subunit IIB: Well-lithified coralgal frameworks associated with skeletal packstone/​grainstone to floatstone including Halimeda segments, shell fragments, scarce bryozoan skeletons, and volcanic elements ranging from fine sand-sized grains to pebble-sized gravels. Coral assemblages include branching Pocillopora, Acropora, Porites, and Pavona; tabular Acropora; encrusting Montipora, Porites, Leptastrea, and agaricids; and massive Porites and faviids. Coral colonies are usually encrusted with nongeniculate coralline algae, locally associated with vermetid gastropods and serpulids, and with microbialites (massive laminated fabrics overlain by thrombolitic accretions). In situ corals and coral clasts usually exhibit bioerosion features and are thickly encrusted with nongeniculate coralline algae, vermetid gastropods, and microbialites. Corals are diagenetically altered, indicating episodic subaerial exposure. Abundant solution cavities are filled with several generations of infillings, some of them displaying a geopetal pattern, consisting of well-lithified pale brownish limestone, weakly lithified dark brown skeletal sand, and dark gray volcaniclastic skeletal sand and silt overlain by microbialite crusts. Some cavities are locally rimmed with multiple generations of cement crusts.
  • Subunit IIC: Beds mostly composed of rubbles and gravels primarily composed of coral clasts, limestone clasts, basalt pebbles, and reworked coral colonies (Porites and robust branching Acropora). Coral clasts include robust branching or tabular colonies of Acropora (A. humilis?), robust branching colonies of Pocillopora, branching colonies of Acropora and Porites, encrusting colonies of Montipora, and encrusting and submassive Porites. Associated skeletal grains include Halimeda segments and bryozoans. Clasts usually display abundant traces of bioerosion; some clasts are encrusted with coralline algae, whereas other clasts are partly embedded in Halimeda packstone.